Mammographic density (MD) is a strong risk factor for breast cancer and is also a highly heritable trait with ~60-70% of the variance in due to genetic factors based on twin studies. MD is also higher in families with a strong history of breast cancer. Genome wide association studies (GWAS), which focus on common genetic variants, have identified several single nucleotide polymorphisms (SNPs) associated with MD. However, these SNPs explain a very small fraction of the variance of MD suggesting many other genes are involved. Linkage studies have identified some loci that may be associated with this trait, but no genes have been mapped yet by linkage analysis. Thus, the vast majority of the heritability of mammographic density remains unexplained and is likely due to rare variants. We have recently identified an association between higher mammographic density and Ashkenazi Jewish ancestry. Since Ashkenazi Jews are a founder population genetic mapping in this population may have several advantages. In particular, Ashkenazi Jews share extensive chromosomal segments that are identical by descent (IBD) due to a small number of founders. We propose to leverage the IBD segments to help map loci for mammographic density. In particular, we plan to search for regions that are IBD among Ashkenazi Jewish women in the top quintile of age and body mass index (BMI)-adjusted mammographic density for shared IBD segments and compare these to Ashkenazi Jewish women in the lowest quintile of age and BMI- adjusted mammographic density. We will then select the top regions and sequence them in the high density women and low density comparison group to identify the genetic variants most likely to be associated with this trait.